[fix] --parallel has precedence over COMPOSE_PARALLEL_LIMIT

[ndeloof]

What I did
As we check --parallel flag has been set, we have to check the "compose" top-level command, not the current one. Surprisingly, cobra ignores request to check a changed flag for undefined flag, otherwise we would have detected this earlier

also groups COMPOSE_* definition as constants as boy scout "clean code" principle

Related issue
fixes #10547

(not mandatory) A picture of a cute animal, if possible in relation to what you did

[codecov]

Codecov Report

Patch coverage: 61.11% and no project coverage change.

Comparison is base (77dc9b5) 59.59% compared to head (3340f7f) 59.59%.

Additional details and impacted files

@@           Coverage Diff           @@
##               v2   #10566   +/-   ##
=======================================
  Coverage   59.59%   59.59%           
=======================================
  Files         107      107           
  Lines        9341     9349    +8     
=======================================
+ Hits         5567     5572    +5     
- Misses       3199     3201    +2     
- Partials      575      576    +1     

Impacted Files	Coverage Δ
cmd/compose/up.go	70.00% <50.00%> (ø)
cmd/compose/compose.go	73.77% <53.84%> (-0.31%)	⬇️
cmd/compose/down.go	78.72% <100.00%> (ø)
cmd/compose/kill.go	48.14% <100.00%> (ø)
cmd/compose/run.go	68.47% <100.00%> (ø)

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[glours]

LGTM